1. Field of the Invention
The present invention relates to a seed layer structure for a platinum manganese pinning layer of a spin valve sensor and more particularly to a seed layer structure that promotes a higher exchange coupling field Hex and a higher pinning field Hp between the pinning layer and a pinned layer of the spin valve sensor.
2. Description of the Related Art
A spin valve sensor is employed by a read head for sensing magnetic signal fields from a moving magnetic medium, such as a rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning a magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) which is an exposed surface of the sensor that faces-the magnetic disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or bias point position in response to positive and negative magnetic field signals from a rotating magnetic disk. The quiescent position, which is typically parallel to the ABS, is the position of the magnetic moment of the free layer with the sense current conducted through the sensor in the absence of signal fields.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered at the interfaces of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering changes the resistance of the spin valve sensor as a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in the resistance of the sensor as the magnetic moment of the free layer rotates from a position parallel with respect to the magnetic moment of the pinned layer to an antiparallel position with respect thereto and R is the resistance of the sensor when the magnetic moments are parallel.
A read head in a magnetic disk drive of a computer includes the spin valve sensor, nonconductive nonmagnetic first and second read gap layers and ferromagnetic first and second shield layers. The spin valve sensor is located between the first and second read gap layers and the first and second read gap layers are located between the first and second shield layers. In the construction of the read head the first shield layer is first formed followed by formation of the first read gap layer, the spin valve sensor, the second read gap layer and the second shield layer. Spin valve sensors are classified as a top or a bottom spin valve sensor depending upon whether the pinning layer is located at the bottom of the sensor next to the first read gap layer or at the top of the sensor closer to the second read gap layer. Spin valve sensors are further classified as simple pinned or antiparallel pinned depending upon whether the pinned layer structure is one or more ferromagnetic layers with a unidirectional magnetic moment or a pair of ferromagnetic layers that are separated by a coupling layer with antiparallel magnetic moments.
Because of the interfacing of the pinning and pinned layers the pinned layer is exchange coupled to the pinning layer. A unidirectional orientation of the magnetic spins of the pinning layer pins the magnetic moment of the pinned layer in the same direction. The orientation of the magnetic spins of the pinning layer are set by applying heat at or above a blocking temperature of the material of the pinning layer in the presence of a field that is directed perpendicular to the ABS. The blocking temperature is the temperature at which all of the magnetic spins of the pinning layer are free to rotate in response to an applied field. During the setting, the magnetic moment of the pinned layer is oriented parallel to the applied field and the magnetic spins of the pinning layer follow this orientation. When the heat is reduced below the blocking temperature the magnetic spins of the pinning layer pin the orientation of the magnetic moment of the pinned layer. The pinning function is effective as long as the temperature remains substantially below the blocking temperature.
Nickel oxide (NiO) is a desirable material for the aforementioned pinning layer structure in a bottom spin valve. Unfortunately, nickel oxide (NiO) has a relatively low blocking temperature which is about 220xc2x0 C. In a magnetic disk drive the operating temperature may exceed 150xc2x0 C. A portion of the magnetic spins of the nickel oxide (NiO) pinning layer rotate below the blocking temperature because of a blocking temperature distribution below the blocking temperature where portions of the magnetic spins of the pinning layer commence to rotate. Accordingly, a portion of the magnetic spins of the nickel oxide (NiO) pinning layer can rotate at operating temperatures in the presence of a magnetic field, such as a signal field from the rotating magnetic disk, the write field from the write head or an unwanted electric static discharge (ESD) caused by contact with a statically charged object. The problem is exacerbated when the slider contacts an asperity on the magnetic disk which can raise the temperature above the disk drive operating temperature.
In the presence of some magnetic fields the magnetic moment of the pinned layer can be rotated antiparallel to the pinned direction. The question then is whether the magnetic moment of the pinned layer will return to the pinned direction when the magnetic field is relaxed. This depends upon the strength of the exchange coupling field and the coercivity of the pinned layer. If the coercivity of the pinned layer exceeds the exchange coupling field, the exchange coupling field will not be strong enough to bring the magnetic moment of the pinned layer back to the original pinned direction. Until the magnetic spins of the pinning layer are reset the read head is rendered inoperative. Accordingly, there is a strong felt need to increase the exchange coupling field between the pinning layer and the pinned layer so that the sensor has improved thermal stability.
Another parameter that indicates the performance of the pinning of the pinned layer is the pinning field Hp between the pinning and pinned layers. The pinning field, which is somewhat dependent upon the exchange coupling field Hex, is the applied field at which the magnetic moment of the pinned layer commences to rotate in a substantial manner. If the pinning field Hp is low the performance of the pinned layer structure relative to the free layer will be degraded. The exchange coupling field Hex and the pinning field Hp will be discussed in more detail in the detailed description.
A desirable antiferromagnetic pinning layer material is platinum manganese (PtMn) since it has a higher blocking temperature than nickel oxide (NiO) and it will perform satisfactorily with less thickness than nickel oxide (NiO). The higher blocking temperature improves thermal stability and the thinner layer improves the read gap. While nickel oxide (NiO) has a blocking temperature of about 220xc2x0 C. and requires a thickness of about 425 xc3x85, platinum manganese (PtMn) has a blocking temperature of a about 350xc2x0 C. and requires a thickness of about 175 xc3x85. Unfortunately, platinum manganese (PtMn) has demonstrated a low exchange coupling field Hex and a low pinning field Hp in bottom spin valves. If these parameters could be increased, platinum manganese (PtMn) would be a very desirable material for pinning layers in spin valve sensors.
The present invention provides a novel seed layer structure for a platinum manganese (PtMn) pinning layer that significantly increases the exchange coupling field Hex and the pinning field Hp between the pinning layer and pinned layer structure and makes the platinum manganese (PtMn) pinning layer desirable for use in bottom spin valves. The seed layer structure comprises first, second and third seed layers wherein the first seed layer is aluminum oxide (Al2O3), the second seed layer is nickel manganese oxide (NiMnO) and the third seed layer is tantalum (Ta). The first seed layer may comprise the first read gap layer, which is typically aluminum oxide (Al2O3), or may be a separate aluminum oxide (Al2O3) layer formed on the first read gap layer. The second seed layer is located between and interfaces the first and third seed layers and the third seed layer interfaces the pinning layer structure. The seed layer structure is especially useful in AP pinned spin valves because of the noted improvement of an antiferromagnetic coupling between first and second AP pinned layers through a ruthenium AP coupling layer. Because of the improved pinning field the thickness of platinum manganese (PtMn) may be decreased from about 175 xc3x85 to about 140 xc3x85.
Other significant advantages provided by the present seed layer structure is an improved magnetoresistive coefficient dr/R, ability to reset orientation of the magnetic spins of the pinning layer at a lower temperature and an option of employing a negative ferromagnetic coupling field Hf between the pinned layer structure and the free layer structure. A typical reset of a platinum manganese (PtMn) pinning layer requires annealing at a temperature of about 250xc2x0 C. for five hours in the presence of a field perpendicular to the ABS. With the present invention the reset can be accomplished by annealing at a temperature of about 220xc2x0 C. for five hours in the presence of a field perpendicular to the ABS. The advantages are that less magnetoresistive coefficient dr/R will be lost during the annealing and the orientation of the magnetic moments of the shield layers, which is perpendicular to the orientation of the magnetic moment of the pinned layer, will be less affected. By appropriately varying the copper thickness of the free layer a negative ferromagnetic coupling field Hf can be achieved. It is important that the orientation of the magnetic moment of the free layer be maintained substantially parallel to the ABS in a quiescent state (without the application of signal fields from a rotating disk). Forces affecting this orientation are sense current fields from all the conductive layers of the spin valve sensor except the free layer, a net demagnetization field from the pinned layer and the ferromagnetic coupling field from the pinned layer. If the ferromagnetic coupling field is negative it can be employed to offset the net demagnetization field from the pinned layer and/or the sense current field for promoting the desired orientation of the magnetic moment of the free layer.
A further significant advantage provided by the present seed layer structure is the predictability of magnetic heads constructed on different wafers. It is believed that the present seed layer structure has a more repeatable and uniformly defined surface that permits the pinning layer formed thereon to perform in the same manner from wafer to wafer. It should be noted that the improved seed layer structure is also usable with other antiferromagnetic materials for pinning layers, such as nickel manganese (NiMn), iridium manganese (IrMn) and iron manganese (FeMn).
An object of the present invention is to provide a seed layer structure for a pinning layer that improves the performance and predictability of a spin valve sensor.
Another object is to provide an improved seed layer structure for a platinum manganese (PtMn) pinning layer that makes the platinum manganese (PtMn) pinning layer highly desirable for use in a bottom spin valve.
A further object is to provide a seed layer structure for a platinum manganese (PtMn) pinning layer which improves the magnetoresistive coefficient dr/R of a spin valve sensor, improves the texture of all layers constructed on the seed layer structure, enables a reset of the pinning layer at a lower temperature, permits employment of a negative ferromagnetic coupling field for balancing the magnetic moment of the free layer and results in predictable performance of magnetic heads from wafer to wafer.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.